Clock divider and method for dividing clock signal in DLL circuit

ABSTRACT

A clock divider of a DLL circuit for generating an internal clock signal synchronized to an external clock signal is disclosed which includes a first clock divider for generating a first clock signal dividing a clock signal having the same period with the external clock signal, a second clock divider for generating a second clock signal and a third clock signal by dividing the first clock signal, a selection signal generator for generating a selection signal in response to a plurality control signals and a clock signal selector for selectively outputting the second clock signal or third clock signal in response to the selection signal.

TECHNICAL FIELD

[0001] A clock divider and a method for dividing a clock signal in a DLL circuit are disclosed for a semiconductor memory device.

DESCRIPTION OF THE RELATED ART

[0002] Generally, clock signals are employed as a reference in order to set an operation timing in a system or a circuit and, clock signals are also employed to secure a speedy operation without any error. When a clock signal inputted from an external circuit is used in a circuit, a time delay, i.e., a clock skew is generated due to an internal circuit. A delay locked loop (DLL) circuit is used in order that the internal clock signal has the same phase with the external clock signal by compensating the clock skew.

[0003] Important factors required in the DLL are a small area, a low jitter and a speedy locking time. These factors will still require semiconductor memory devices of the future, which will also require low power consumption and speedy operation. There is merit in that the DLL circuit is less influenced by noise than a phase locked loop (PLL), so that the DLL circuit is employed in a synchronous semiconductor memory device, such as a DDR SDRAM (double data rate synchronous DRAM) or the like. A DLL circuit of a register type is mainly employed in many kinds of DLL circuits.

[0004]FIG. 1 is a block diagram illustrating a conventional register control type DLL circuit of a typical DDR SDRAM.

[0005] The conventional register control type DLL circuit of the typical DDR SDRAM includes a clock buffer 110, a first clock divider 130, a delay circuit 150, a clock multiplexer 170, a second clock divider 190, a delay model 210, a phase comparator 230 and a delay controller 250.

[0006] The clock buffer 110 converts a voltage level of an external clock signal CLK and an external clock inversion signal CLKB having a high frequency inputted from an external circuit into a power supply voltage level VDD. The first clock divider 130 outputs a reference clock of a low frequency by dividing a high frequency clock signal CLKD of a VDD level by n, wherein n is a positive integer and, generally, n is 4. The delay circuit 150 delays the high frequency clock signal CLKD of the VDD level as much as a predetermined delay and outputs a delayed clock signal to the clock multiplexer 170. The delay circuit 150 includes a plurality of delay units forming a delay chain and a shift register for controlling the plurality of delay units. Each delay unit consists of an NAND gate and an inverter. The multiplexer 170 outputs the delayed clock signal OUTPUT DLL_CLK to an external circuit and to the second clock divider 190. The second divider 190 divides the delayed clock signal outputted from the multiplexer 170 by n, wherein n is a positive integer and, generally, n is 4. The delay model 210 is configured in order that a feedback signal has an identical delay condition to the real clock signal path.

[0007] Subsequently, the phase comparator 230 compares a phase of the feedback signal outputted from the delay model 210 with that of the reference clock signal REF. The delay controller 250 outputs shift control signals SR and SL for controlling a shift direction of the shift register in the delay circuit 150 and a delay locking signal representing that the delay locking is achieved in response to control signals EARLY and LATE outputted from the phase comparator 230.

[0008] The delay model 210 includes a dummy clock buffer, a dummy output buffer and a dummy load in order to have identical delay time to a delay time generated in a real clock signal path and is called as a replica circuit. Since the delay circuit 150, the delay controller 250 and the phase comparator 230 delay the external clock signal CLK as much as desired, they are called as a delay unit.

[0009] Since the delay model 210 includes a dummy clock buffer, a dummy output buffer and a dummy load, a delay time of the clock signal generated from the clock buffer, an output buffer and a load can be compensated. At this time, since the external clock signal is not synchronized with the internal clock signal, a repeated delay operation for synchronizing the external clock signal with the internal clock signal is carried out in the delay circuit 150. Since the delay amount of the delay model 210 cannot be changed for achieving locking, total delay amount has to be adjusted in the delay circuit 150. A condition for achieving locking is as follows:

DD+RR=nT   (Eq. 1)

[0010] Where, DD is a delay amount of the delay circuit 150, RR is a delay amount of the delay model 210, T is a period of the external clock signal and n is integer, e.g., 1 or 2.

DD=nT−RR   (Eq. 2)

[0011] Accordingly, an output DLL clock OUTPUT_DLL_CLK is outputted by repeated delaying the high frequency clock signal CLKD as much as DD, which is a delay amount repeatedly adjusted in the delay circuit 150. That is, a negative delay, preceding as much as RR compared with the external clock signal, is achieved in the DLL circuit.

[0012]FIG. 2A is a timing diagram illustrating one period (1T) based dividing of the clock signal capable of being used in a low frequency band and FIG. 2B is a timing diagram illustrating two period (2T) based dividing of the clock signal capable of being used in a high frequency band.

[0013] Referring to FIG. 2A, since a rising edge of a feedback clock signal, which is compared in the low frequency band, is later than that of the reference clock signal REF, locking can be achieved by repeatedly increase the delay amount in the delay circuit 150. In this case, since a pulse width of the divided clock signal corresponds to one period of the external clock signal, it is called as the 1T based dividing.

[0014] Referring to FIG. 2B, since a rising edge of a feedback clock signal, which is compared in the high frequency band, is faster than that of the reference clock signal REF, locking can not be achieved only by repeatedly increasing the delay amount in the delay circuit 150. Accordingly, as a pulse width of the divided clock signal corresponds to two periods of the external clock signal, the locking can be achieved. Namely, since the pulse width of the divided clock signal corresponds to two periods of the external clock signal, it is called as the 2T based dividing.

[0015]FIG. 3A is a circuit diagram illustrating a conventional four-dividing circuit for one period based dividing, which a pulse width is not adjustable, structured in the clock signal divider, e.g., the first and the second clock signal dividers 130 and 190, according to the prior art. FIG. 3B is a timing diagram showing an operation of the conventional four-dividing circuit in FIG. 3A.

[0016] If the clock signal CLKD inputted through the clock buffer 110 is inputted to the first dividing unit 310 in response to a DLL enable signal DLL_ENABLE, the clock signal CLKD is divided by two and then the two-divided clock signal DIVIDE_2 is outputted. Thereafter, if the two-divided clock signal DIVIDE_2 is inputted to a second dividing unit 330, a four-divided clock signal DIVIDE_4, which is maintained in a high state one for one period of the external clock signal and then is maintained in a low state, is outputted.

[0017]FIG. 4A is a circuit diagram illustrating a conventional four-dividing circuit for one period based dividing, which a pulse width is adjustable, structured in the clock signal divider, e.g., the first and the second clock signal dividers 130 and 190, according to the prior art. FIG. 4B is a timing diagram showing an operation of the conventional four-dividing circuit in FIG. 4A.

[0018] If the clock signal CLKD inputted through the clock buffer 110 is inputted to a third dividing unit 410 in response to a DLL enable signal DLL_ENABLE, the clock signal CLKD is divided by two and then the two-divided clock signal DIVIDE_2 is outputted. Thereafter, if the two-divided clock signal DIVIDE_2 is inputted to a fourth dividing unit 430, the two-divided clock signal DIVIDE_2 is divided again by 2. Namely, an output clock signal DIVIDE_4 of the fourth dividing unit 430 is maintained in a high state for two periods of the external clock signal and then is maintained in a low state.

[0019] As shown in the timing diagrams in FIGS. 3 and 4, however, the pulse width of the divided clock signal cannot be changed according to the frequency bands.

[0020] Recently, when the input clock signal is a high frequency, the two periods based dividing is carried out in order to secure an operation in the high frequency band. Even if the two periods based dividing has a good performance in the high frequency band, a serious noise is caused in the low frequency band, so that a malfunction of the semiconductor memory device is frequently caused.

[0021] Also, when one period based dividing is carried out in order to reduce the noise, it is difficult that an operation frequency becomes over 100 MHz to 133 MHz and an area of the semiconductor memory device is increased due to a lot of delay circuits.

SUMMARY OF THE DISCLOSURE

[0022] A clock divider capable of reducing jitter due to a noise of an external power supply voltage and reducing a size of the DLL circuit. Further, a method for dividing a clock signal in a DLL circuit of a semiconductor memory device is also disclosed.

[0023] A disclosed clock divider of a DLL circuit for generating an internal clock signal synchronized to an external clock signal in a semiconductor memory device, comprises: a first clock dividing means for generating a first clock signal dividing a clock signal having the same period with the external clock signal; a second clock dividing means for generating a second clock signal and a third clock signal by dividing the first clock signal outputted from the first clock dividing means; a selection signal generation means for generating a selection signal in response to a plurality control signals; and a clock signal selection means for selectively outputting the second clock signal or third clock signal in response to the selection signal.

[0024] A disclosed method for dividing a clock signal of a DLL circuit for generating an internal clock signal synchronized to an external clock signal in a semiconductor memory device comprises: generating a first clock signal by dividing a clock signal having the same period with the external clock signal; generating a second clock signal and a third clock signal by dividing the first clock signal; generating a selection signal by receiving a long locking signal generated when a high frequency clock signal is inputted to the DLL circuit, a DLL enable signal generated when the DLL is enabled and a dividing clock signal selection enable signal to be a logic high state only for initial four cycles after the DLL circuit is turned on; and selectively outputting the second clock signal for a high frequency clock signal or the third clock signal for a low frequency clock signal in response to the selection signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The above and other features of the disclosed clock divider and method will become apparent from the following description of preferred embodiments taken in conjunction with the accompanying drawings, wherein:

[0026]FIG. 1 is a block diagram illustrating a DLL circuit of a register control type of a typical DDR SDRAM;

[0027]FIG. 2A is a timing diagram illustrating one period (1T) based dividing of the clock signal capable of being used in a low frequency band;

[0028]FIG. 2B is a timing diagram illustrating two period (2T) based dividing of the clock signal capable of being used in a high frequency band;

[0029]FIG. 3A is a circuit diagram illustrating a conventional four-dividing circuit for one period based dividing of a clock signal;

[0030]FIG. 3B is a timing diagram showing an operation of the conventional four-dividing circuit of FIG. 3A;

[0031]FIG. 4A is a circuit diagram illustrating another conventional four-dividing circuit for one period based dividing of a clock signal;

[0032]FIG. 4B is a timing diagram showing an operation of the conventional four-dividing circuit of FIG. 4A;

[0033]FIG. 5 is a block diagram illustrating a four-dividing clock divider capable of adjusting a pulse width;

[0034]FIG. 6A is a circuit diagram illustrating a four-dividing clock divider capable of adjusting a pulse width in accordance with a preferred embodiment;

[0035]FIG. 6B is a timing diagram illustrating an operation of the four-dividing clock divider of FIG. 6A;

[0036]FIG. 7 is a circuit diagram illustrating the selection signal generator of FIG. 5;

[0037]FIG. 8 is a timing diagram showing an operation of the four-dividing clock divider for a high frequency; and

[0038]FIG. 9 is a timing diagram showing an operation of the four-dividing clock divider for a low frequency.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0039] Hereinafter, a clock divider in a DLL circuit capable of adjusting a pulse width will be described in detail referring to the accompanying drawings.

[0040]FIG. 5 is a block diagram illustrating a four-dividing clock divider capable of adjusting a pulse width.

[0041] The four-dividing clock divider includes a first clock divider 510, a second clock divider 530, a selection signal generator 550 and a clock selector 570. The first clock divider 510 is a two-dividing circuit and generates a first clock signal by receiving a clock signal CLKD having an identical period with an external clock, wherein the first clock signal is a divided clock signal dividing an input clock signal by 2, and the second clock divider 530 generates 1T-based dividing clock signal and 2T-based dividing clock signal. The selection signal generator 550 generates a selection signal for selecting one of the 1T-based dividing clock signal and the 2T-based dividing clock signal and the clock selector 570 outputs one of the 1T-based dividing clock signal and the 2T-based dividing clock signal in response to the selection signal.

[0042] The four-dividing clock divider can be applied to the first and second clock divider 130 and 190 in FIG. 1. For the sake of convenience, an example applied the first clock driver 130 will be described.

[0043] The first clock divider 510 receives the clock signal CLKD having an identical period to that of the external clock signal and generates two-dividing clock signal. The two-dividing clock signal is maintained in a first logic state for one period of the clock signal CLKD and in a second logic state for one period of the clock signal CLKD.

[0044] The second clock divider 530 receives the two-dividing clock signal and generates the 1T-based dividing clock signal and the 2T-based dividing clock signal. The 1T-based dividing clock signal is maintained in a first logic state for one period of the clock signal CLKD and in a second logic state for three periods of the clock signal CLKD and the 2T-based dividing clock signal is maintained in a first logic state for two periods of the clock signal CLKD and in a second logic state for two periods of the clock signal CLKD.

[0045] The selection signal generator 550 receives a long locking signal LONG_LOCK, a DLL enable signal DLL_ENABLE and a dividing clock selection enable signal TCK_SET ENABLE and generates the selection signal for selecting one of the 1T_based dividing clock signal and the 2T_based dividing clock signal according to a frequency of the external clock signal. The clock divider 570 selectively outputs one of the 1T_based dividing clock signal and the 2T_based dividing clock signal in response to the selection signal.

[0046]FIG. 6A is a circuit diagram illustrating a four-dividing clock divider capable of adjusting a pulse width in accordance with a preferred embodiment.

[0047] The first clock divider 510 receives the clock signal CLKD and the DLL enable signal DLL_ENABLE and outputs the two-dividing clock signal DIVIDE_2. The second clock driver 530 receives the two-dividing clock signal DIVIDE_2 and generates the 1T_based dividing clock signal and the 2T_based dividing clock signal, which correspond to four-dividing clock signal. The 1T_based dividing clock signal or the 2T_based dividing clock signal generated from the second clock driver 530 is selectively outputted through a clock selector 650 in response to the selection signal TCK_CTRL generated from the selection signal selector 550 shown in FIG. 5.

[0048]FIG. 6B is a timing diagram illustrating an operation of the four-dividing clock divider in FIG. 6A.

[0049] When the selection signal TCK_CTRL is a logic high state, the 1T_based dividing clock signal is outputted and, when the selection signal TCK_CTRL is a logic low state, the 2T_based dividing clock signal is outputted.

[0050]FIG. 7 is a circuit diagram illustrating the selection signal generator 550 in FIG. 5.

[0051] As shown, the selection signal generator 550 receives the long locking signal LONG_LOCK, the DLL enable signal DLL_ENABLE and the dividing clock signal selection enable signal TCK_SET ENABLE and generates the selection signal TCK_CTRL. The long locking signal LONG_LOCK is a deactivated signal outputted from a phase comparator 230 in FIG. 1 when the delay amount DD+RR in Eq. 1 becomes over one period of the external clock signal. The DLL enable signal DLL_ENABLE is generated when the DLL is enabled. The dividing clock signal selection enable signal TCK_SET ENABLE becomes a logic high state only for initial four cycles after the DLL is turned on. Herein, since the dividing clock signal selection enable signal TCK_SET ENABLE may be easily generated by using a D flip/flop and a latch in or outside the DLL, a circuit for generating the dividing clock signal selection enable signal TCK_SET ENABLE is not described.

[0052] When the dividing clock signal selection enable signal TCK_SET ENABLE is a logic high state, the selection signal generator 550 determines whether the selection signal TCK_CTRL is a logic high state or a logic low state.

[0053] Since the delay amount DD+RR in Eq. 1 becomes over one period of the external clock signal in a high frequency band, the long locking signal LONG_LOCK is generated from the phase comparator 230 in FIG. 1. If the long locking signal LONG_LOCK is enabled with a logic low state, an output NET2 of the first NAND gate ND1 becomes a logic high state. For the initial four cycles of the DLL, since the dividing clock selection enable signal TCK_SET ENABLE becomes a logic high state, an output NET4 of a third NAND gate ND3 becomes a logic low state. Since the DLL enable signal DLL_ENABLE and the dividing clock selection enable signal TCK_SET ENABLE are a logic high state, an output NET3 of a second NAND gate ND2 becomes a logic low state and an output NET4 of a fourth NAND gate ND5 becomes a logic high state. Since an output of the fifth NAND gate ND becomes a logic high state by receiving the output NET of a logic low state, an output of a sixth NAND gate ND6 becomes a logic low state. Therefore, the dividing clock selection enable signal TCK_SET ENABLE is generated with a logic low state through inverters INV1 and INV2.

[0054] Since the delay amount DD+RR in Eq. 1 becomes below one period of the external clock signal in a low frequency band, the long locking signal LONG_LOCK is not generated from the phase comparator 230 in FIG. 1. If the long locking signal LONG_LOCK is disabled with a logic high state, the output NET2 of the first NAND gate ND1 becomes a logic low state. For the initial four cycles of the DLL, since the dividing clock selection enable signal TCK_SET ENABLE becomes a logic high state, the output NET4 of the third NAND gate ND3 becomes a logic high state. Since the DLL enable signal DLL_ENABLE and the dividing clock selection enable signal TCK_SET ENABLE are a logic high state, an output NET3 of a second NAND gate ND2 becomes a logic high state and an output NET4 of a fourth NAND gate ND5 becomes a low state. Since the output of the sixth NAND gate ND6 becomes a logic high state by receiving the output NET5 of a logic low state, the dividing clock selection enable signal TCK_SET ENABLE is generated with a logic high state through inverters INV1 and INV2.

[0055]FIGS. 8 and 9 are timing diagrams showing an operation of the four-dividing clock divider.

[0056]FIG. 8 is a timing diagram showing an operation of the four-dividing clock divider for a high frequency and FIG. 9 is a timing diagram showing an operation of the four-dividing clock divider for a low frequency.

[0057] As shown in FIG. 8, the selection signal TCK_CTRL is transited to a logic low state when the long locking signal LONG_LOCK signal is generated for the high frequency band.

[0058] As shown in FIG. 9, the selection signal TCK_CTRL is maintained in a logic high state since the long locking signal LONG_LOCK signal is not generated for the low frequency band.

[0059] In FIGS. 8 and 9, a reference signal is an inverted output signal selectively outputted from the 1T-based dividing clock signal and the 2T-based dividing clock signal through the clock selector 570 in response to the selection signal TCK_CTRL.

[0060] Accordingly, since the four-dividing clock divider can be applied to a wide frequency band, a size of the DLL can be reduced. Also, since the four-dividing clock divider is strong for a noise of the external power supply voltage, the jitter is reduced by ⅓.

[0061] While the disclosed divider and method of dividing a clock signal have been described with respect to the particular embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of this disclosure which is limited only by the following claims. 

What is claimed is:
 1. A clock divider in a DLL circuit for generating an internal clock signal synchronized to an external clock signal, the clock divider comprising: a first clock dividing means for generating a first clock signal dividing a clock signal having the same period with the external clock signal; a second clock dividing means for generating a second clock signal and a third clock signal by dividing the first clock signal outputted from the first clock dividing means; a selection signal generation means for generating a selection signal in response to a plurality control signals; and a clock signal selection means for selectively outputting the second clock signal or third clock signal in response to the selection signal.
 2. The clock divider as recited in claim 1, wherein the first clock dividing means comprises a two-dividing circuit.
 3. The clock divider as recited in claim 2, wherein one period of the first clock signal corresponds to two periods of the clock signal.
 4. The clock divider as recited in claim 3, wherein the second clock signal is one period based dividing clock signal, whose one period corresponds to four periods of the clock signal, maintaining a first logic state for one period of the clock signal and a second logic state for three periods of the clock signal.
 5. The clock divider as recited in claim 4, wherein the third clock signal is two periods based dividing clock signal, whose one period corresponds to four periods of the clock signal, maintaining a first logic state for two periods of the clock signal and a second logic state for two periods of the clock signal.
 6. The clock divider as recited in claim 5, wherein the plurality of control signals comprises a long locking signal generated when a high frequency clock signal is inputted to the DLL circuit, a DLL enable signal generated when the DLL is enabled and a dividing clock signal selection enable signal to be a logic high state only for initial four cycles after the DLL circuit is turned on.
 7. The clock divider as recited in claim 6, wherein the selection signal is enabled with a second logic state for a high frequency clock signal and disabled with a first logic state for a low frequency clock signal.
 8. The clock divider as recited in claim 7, wherein the clock selection means outputs the second clock signal in response to the selection signal of the second logic state and the third clock signal in response to the selection signal of the first logic state.
 9. A method for dividing a clock signal in a DLL circuit for generating an internal clock signal synchronized to an external clock signal, the method comprising: generating a first clock signal by dividing a clock signal having the same period with the external clock signal; generating a second clock signal and a third clock signal by dividing the first clock signal; generating a selection signal by receiving a long locking signal generated when a high frequency clock signal is inputted to the DLL circuit, a DLL enable signal generated when the DLL is enabled and a dividing clock signal selection. enable signal to be a logic high state only for initial four cycles after the DLL circuit is turned on; and selectively outputting the second clock signal for a high frequency clock signal or the third clock signal for a low frequency clock signal in response to the selection signal.
 10. The method as recited in claim 9, wherein one period of the first clock signal corresponds to two periods of the clock signal.
 11. The method as recited in claim 10, wherein the second clock signal is one period based dividing clock signal, whose one period corresponds to four periods of the clock signal, maintaining a first logic state for one period of the clock signal and a second logic state for tree periods of the clocks signal.
 12. The method as recited in claim 11, wherein the third clock signal is two periods based dividing clock signal, whose one period corresponds to four periods of the clock signal, maintaining a first logic state for two periods of the clock signal and a second logic state for two periods of the clock signal. 